For a party that believes in keeping the state out of people's private lives, the Conservatives get awfully excited about gay marriage.

You'd think what happens in the bedroom is precisely the kind of thing the Tories would view as a matter of private morality.

But for many older Tories, today's vote on gay marriage is a sign of how modern life is starting to leave them behind.

Fifty years ago, homosexuality was illegal, now -- barring a sudden capitulation by Downing Street -- marriage will be "equalised" (to use the kind of language that so angers David Cameron's critics.)

And they are certainly angry. David Burrowes, the Conservative backbencher leading the Commons revolt, says the policy is "dangerous", though it's not clear exactly what danger married homosexuals present.

More than half the parliamentary Conservative party could either vote against gay marriage, or abstain.

The problem is that this is as much a matter of principle for the Prime Minister as it is for those who oppose him.

Those close to David Cameron say he really believes in gay marriage, and in fact thinks it's a rather Conservative idea -- a couple making a legal commitment to each other.

He feels it's down to his government to resolve the issue, even if that damages his relations with many grassroots Tories.

The David Cameron of 2005 would have relished this fight -- a chance to make a public display of how far his party has travelled.

He would have done so, relatively confident that come 2015, those disgruntled Tories would still support him, as they had nowhere else to go.

Return to 2013, though, and you quickly realise the Prime Minister is taking a significant gamble.

Those alienated Tories, voters and activists, could march towards UKIP. Their defection, should it happen, would make the task of winning the 2015 election even harder.

But Cameron is trapped. If he abandons the gay marriage vote, he'll lose the potential support of younger, metropolitan voters. The kind who, like Cameron, think it's ludicrous this issue hasn't already been resolved. The kind who, ultimately, may well decide the next election.

Gay marriage will be passed by the Commons. But David Cameron will be relying on Labour votes as much as his Lib Dem coalition partners. And most of the right-wing goodwill he earned with his EU referendum pledge will disappear.